It has been a common problem that water-based inks generally have poor water-fastness. Their solubility in water makes them effective as ink jet inks but this same property lessens their effectiveness in security systems.
While some red-luminescent inks provide particularly effective security features and it would be desirable to utilize them in ink jet printing devices, improvements are necessary to give them more fastness in the presence of water.
The art has provided a number of very interesting security inks based on lanthanide metal compounds and has developed a variety of security systems employing them. For example, In U.S. Pat. No. 5,837,042, Lent, et al., describe invisible fluorescent jet inks said to be suitable for producing security markings on documents and other articles. The jet ink compositions comprise a fluorescent colorant, an ink carrier, and optionally one or more binder resins. The markings are invisible to the unaided eye and are visible only when excited by ultraviolet light. The colorant comprises a rare earth metal and a chelating ligand, is excitable by ultraviolet light having a wavelength of from about 275 nm to about 400 nm, and fluoresces at a wavelength of from about 550 nm to about 700 nm, with the proviso that when the rare earth metal is europium, dysprosium, or terbium, the chelating ligand is not dibenzoylmethane. Also described is a method of identifying objects comprising providing a security marking as described above, exciting the marking and reading the fluorescent emission.
In another, U.S. Pat. No. 6,902,265, Critelli, et al., describe a method for printing high-information-density, machine-readable composite images. They teach printing machine-readable light-colored or invisible, but luminescent, postage-evidencing symbology and dark, other readable postage information, preferably in a single print operation, to provide large amounts of information without diminishing read rates. The resulting combination image can be printed independent of substrate material to enable the provision of a high information density without obscuring any one component. The postage-evidencing symbology is printed in large-format and can include redundant information within an IBI image or between an IBI and visible dark image for a variety of practical purposes. In a preferred form, the images are printed using conventional ink jet printers using water-based inks. In one embodiment of this type, the images are printed with a conventional two-cartridge color ink jet printer by printing postage-evidencing symbology with a single-color ink and printing the visible dark image as a composite dark color from a standard multi-color cartridge. Europium-containing inks can be utilized.
In U.S. Pat. No. 6,905,538 to Auslander, there are described water-based invisible red fluorescent inks provide machine-readable, inverse contrast invisible images and can be printed using conventional ink jet printers. The inks employ rare earth complexed ligand fluorophores having narrow excitation and emission spectra. In one embodiment the images are printed with an ink comprising water and a water-soluble organic fluorescent fluorophore, which when printed and dried on paper, is invisible to the eye and fluoresces in the green to infrared range, e.g., from about 550 to 1200 nm, when irradiated with short wave length UV radiation, e.g., from 230 nm to 280 nm. The highly specific excitation and emission rates, coupled with a high inverse contrast on papers of all colors, makes them particularly useful as ink jet inks for postal and other purposes.
In all of these and other cases of security inks, it is important that images printed with the inks retain their functionality if wetted by rain, spilled liquids or other common causes of wetness. Unfortunately, the factors which make inks easy to print with by ink jet printing, generally detract from the important property of water fastness.
There is a need for postal and security inks with strong fluorescent signals that can be printed by ink jet printing to produce images having a high degree of water fastness.